To provide efficient storage of media files the data is commonly compressed by encoding in a particular format. For example, audio files are commonly encoded in the MP3 format. Mobile devices may decode those files and transmit the data to an output device. Alternatively, the mobile device may transmit the encoded data to the output device for that device to perform the decoding and output. This may be attractive when the mobile device and output device are connected by a radio link as it reduces the data bandwidth required, and also reduces the processing capacity required by the mobile device as there is no requirement to decode the file prior to transmission. The output device may be provided with a dedicated decoding device, allowing the decoding to be performed efficiently.
A common example is the playing of encoded audio files. The encoded data is transmitted to a headset for output to a user. It may be required for the mobile device transmitting the encoded audio file to notify the user of an event by the output of a tone or other audio sign to the user. For example, the mobile device may be configured to audibly indicate to the user when its battery is running low, or if a message arrives. In order to reduce the impact on the user's experience, the tones should be mixed with the audio in a non-invasive manner.
Mixing cannot be performed directly into an audio file in its encoded form, and thus in order for a mobile device to mix tones with audio being sent to a listening device in an encoded format, the encoded file must be decoded, mixed and recoded before transmission. Decoding and encoding (transcoding) of audio files is a processor-intensive activity. For example, decoding and encoding an MP3 encoded file may require approximately 36 MIPS. In general, as the audio quality increases, as do the processing requirements. Transcoding before transmission to allow mixing therefore removes any processing capacity reduction achieved by transmitting encoded files. The processing capacity required for the transcoding also has a significant impact on power consumption, thereby decreasing battery life when playing back audio files.